1. Field of the Invention:
This invention relates to internal combustion engines which employ flexible link transmissions, such as belts. The belts rely on mechanical adherence, or friction, to drive a pulley. Such a belt operates driven devices which receive mechanical power, from the belt. These belts are driven by an internal combustion engine. The belts are used, more specifically, to drive accessories on automobile engines. The belts must be placed under a specific amount of tension for proper operation. Such tension may be applied by a roller, positioned at the end of a movable lever arm.
The lever arm receives an elastic torque, which may be provided by a biasing means in the form of a flat elastomer which is positioned, or sandwiched, between rigid armatures. The elastomer may be adherized, during vulcanization, to the armatures. In use, the elastomer is subjected to shear stress, after initially being subjected to a uniform axial prestress. The axial prestress is transmitted through the friction surface material of the armatures thus damping the elastic tension variations of the belt, by counteracting the variations of shear stress.
2. Description of the Prior Art:
Flexible link transmissions, which rely on frictional, or other mechanical, interaction with a pulley, are, generally, called engine belts. Such belts are essential to the operation of internal combustion engines and are used in automobiles and on stationary engines to drive various accessories, such as alternators, water pumps, air conditioners, power steering pumps, etc.
The accessories can be positively driven by a lubricated articulated chain, as well as by synchronous toothed flexible belts. When utilizing either of such links, as well as when utilizing mechanical adherence belts, the adjustment for wear and for thermal expansions and contractions of the links is performed by a device called a tensioner. The tensioner, generally, operates by means of a cam or by displacement of one of the pulleys of the tensioner. Nevertheless, because of dynamic pulsations applied to the belt during operation, and the longitudinal rigidity of the belt in synchronous drives, the center distances, between pulleys over which the belt runs, should be fixed after adjustment. However, this, possibly, could be at the expense of seeking a desired compensation of expansions and contractions of the engine, as explained in French Patent Application No. 88 13147.
Mechanical adherence transmissions require a permanent tension. In the past, such tension has, generally, been provided by the elasticity of flexible armatures. This is particularly the case in technology employing trapezoidal cross-section or vee belts. With those belts, as well as with those belts described above, but for different reasons, the tensioning device is blocked in a fixed position after preliminary adjustments have been made.
In the most recent generation of flexible links, called multirib belts, the frictional adherence of the belts to parts of the engine is achieved by the employment of a number of inclined faces on the conical sides of the pulleys over which the belts travel.
Flexibility of the armature is acceptable with belt materials having great longitudinal rigidity or rather being inelastic along the length thereof, such as polyester, glass fiber and aramid.
The manufacturers of internal combustion engines have sought a tensioning device with remains elastic in operation and, generally, includes a sheave pulley, supported by an elastic tension lever arm, hereinafter designated a "tensioning roller."
Various tensioning rollers use elastomers to provide elastic tension. Examples of such tension rollers are disclosed in U.S. Pat. No. 4,144,772, which is assigned to The Toro Company, and which discloses a torsion ring embedded in a square block to provide both tension and damping. Great Britain Patent No. 2,070,727, which is assigned to Alan Crosley Pritchard, describes a similar technique, applied to a rocking plate, while European Patent No. 01 57 193, which is assigned to RIV-SKF, applies the technique to a tensioning cam.
U.S. Pat. No. 4,702,727, which is assigned to Daimler-Benz, U.S. Pat. No. 4,472,162, which is assigned to Dyneer Corporation and French Patent No. 25 09408, which is assigned to Litens Automotive, all disclose the utilization of an elastomer for damping purposes through the employment of friction, while an elastic, or biasing, force is exerted by a metal spring. This configuration makes possible the establishment of a tension which provides relatively little variation due to dynamic equilbrium displacements. The reliability, or stability, of the tensions required by the automobile industry may not always be achieved with the damping action provided by elastomer friction, since the tensions provided thereby can vary due to the appearance of fine powder caused by wear or, on the other hand, a softening of surfaces, or creep, because of excessive alternating movements.
For this reason, a ring attached to concentric armatures has been used in the device disclosed in U.S. Pat. No. 3,975,965, which is assigned to Dayco Corporation, and in European Patent No. 01 14 779. The rotation of the interior ring provides regulation of the tension, although the patents differ in the means used to re-establish the tension during maintenance.
The technique of applying radial prestress to an interior ring, for example by hammering on an exterior ring which covers the interior ring, by expanding the interior ring, or by employing both techniques when the interior ring has an intermediate armature, provides rigidity to the interior ring and reduces deformation, since the ring takes on a somewhat conical configuration when twisted. This design also allows additional stress to be applied to an elastomer between the armatures. Such stress, however, is limited by the geometry of the elastic ring, which is used to exert significant tension forces, since the ring occupies a great deal of longitudinal space over the axis of rotation of the lever supporting the tensioning roller. Such a space requirement would require the device to be too thick to be practical.
To avoid rapid variations of tension when variations of the perimeter of the flexible link occur due to wear or creep, good flexibility is necessary. However, such flexibility requires that additional thickness be given to the elastomer ring. That, in turn, increases the volume occupied and the weight of the tensioning device, while the damping is limited by the visco-elastic characteristics of the elastomer material, even if a prestress is applied as mentioned above.
An additional disadvantage of the prior art is that the reduction of the conical deformation occasionally requires the addition of several roller bearings to counteract the deformations of the elastic rings.